The monoterpene alcohols limonen-10-ol and menth-1-en-9-ol have been found to have a wide spectrum of antimicrobial activity against virtually all bacteria, yeast, and fungi. Because they are readily biodegradable, and are considered environmentally safe, they have numerous potential uses as bactericides and fungicides in agricultural, industrial, pharmaceutical, and consumer products. Both compounds inhibit dental plaque and are likely to be used in mouthwash, toothpaste and chewing gum. Uses as topical and enteric pharmaceutical and veterinary products, preservatives for food, medicine, and cosmetics, antiseptics for food processing equipment, and bactericides and fungicides for agriculture, turf, paint, leather, bathrooms, plastics, air filters, lumber, and industrial cutting oils represent only a few of the potential applications of limonen-10-ol and menth-1-en-9-ol. This invention relates to a new and economical method of producing these allylic alcohols.
Both limonen-10-ol and menth-1-en-9-ol have a mild fruity aroma that is easily flavored. Because they are oils they are insoluble in water, but they are readily emulsified (in water). They are poorly soluble in propylene glycol, and are only sparingly soluble in glycerine. They are soluble in alcohol and are miscible in oil.
Prior art exists for the production of menth 1-en-9-ol. For example, the preparation of menth-1-en-9-ol was unintentionally reported by Vijay, Khanna, and Ladwa in the Indian Journal of Chemistry, Section B 1987, 26B (9), 816-822 in a yield of only 15.5% by the hydroboration/ oxidation of carvone. This method is not an economical method of producing commercial quantities of menth-1-en-9-ol. BH.sub.3 is a reagent that easily reduces carbon-carbon double bonds and carbon-oxygen double bonds. In their procedure, the first equivalent of BH.sub.3 reduces the carbonyl moiety of carvone to produce intermediate i which is then followed by the addition of BH.sub.3 to the C(8), C(9) double bond (cf.ii). However, a small amount of intermediate ii rearranges to form unexpected intermediate iii, and generates a hydride ion. The hydride ion then undergoes a displacement reaction to form the intermediate iv, which upon oxidative workup using hydrogen peroxide and sodium hydroxide furnishes a low yield of menth-1-en-9-ol. Modifications of this reaction procedure gave lower yields of menth-1-en-9-ol. It is unlikely that this procedure can be improved upon to increase the production of menth-1-en-9-ol, and precludes the Khanna and Ladwa method of producing commercial quantities of menth-1-en-9-ol economically. Furthermore, their method, that is summarized below, is totally inapplicable to the production of limonen-10-ol. ##STR3##
Brown and Zweifel attempted to achieve the monohydroboration of d-limonene with dibonane, but failed as reported in the Journal of the American Chemical Society, 83, 1244 (1961). Similar unsuccessful results were also described by R. Dulou and Chretien-Dessiere in the Bulletin of the Society Chemistry of France, 9, 1362 (1959). ##STR4##
In the same journal article [JACS, 83, 1244 (1961)] Brown and Zweifel prepared p-menth-l-en-9-ol from limonene in 79% using disiamylborane. While this procedure can be used to produce p-menth-l-en-9-ol on a commercial scale, the cost of the disiamylborane reagent alone makes this a costly process. ##STR5##
The applicants' method of producing menth-1-en-9-ol involves the formation of limonen-10-ol from limonene by the in situ generation of an allylic borate ester, followed by the cleavage of the carbon-boron bond, to produce limonen-10-ol in high yield. Limonen-10-ol was then reduced to menth-1-en-9-ol.
No method of producing limonen-10-ol was uncovered in a search of the literature, but it is recognized that limonene is a cheap material from which numerous natural occurring monoterpenes are derived. For instance limonene is used in the production of perillyl alkanoates as was described by Ansari, Hifzur in Canadian patent 1077959. Carveol can be produced by the oxidation of limonene as was outlined by Bain in U.S. Pat. Nos. 2,863,882 and 3,014,047. Leffingwell (in U.S. Pat. No. 3,538,164) produced limonene-1,2-epoxide from which he derived dihydrocarveol by the addition of small amounts of perchloric acid.
The need to economically supply limonen-10-ol and menth-1-en-9-ol led to this invention that relates to selectively oxidizing Carbon 10 of d-limonene and 1-1limonene to produce limonen-10-ol. Menth-1-en-9-ol is produced by the reduction of limonen-10-ol.
Applicants' invention can be summarized as a method for preparing a monocyclic monoterpene of the formula: ##STR6## wherein R=--C.dbd.CH.sub.2 or --CH--CH.sub.3
The steps in the process of preparation are described in some detail in the following disclosure for converting limonene to the desired products. In the preparation of limonen-10-ol limonene is treated with a strong base to activate the limonene structure. A boron-containing reagent is then added to produce a limonenyl adduct, i.e., a limonene radical having boron added thereto. An oxidizing agent is then added to remove the boron and introduce an hydroxy group at the 10-position of the limonenyl radical, producing two liquid phases by adding water and an organic solvent to the reaction medium; and recovering limonen-10-ol from the organic phase.
If one wishes to produce menth-1-en-9-ol, the limonen-40 -ol is subsequently hydrogenated, reacted with hydrogen gas under subatmospheric pressure in the presence of platinum on activated carbon to produce an essentially pure product of menth-1-en-9-ol in quantitative yield.
The following examples are illustrative of the best mode for carrying out the invention. They are, obviously, not to be construed as limitative of the invention since various other embodiments can readily be evolved in view of the teachings provided herein. In Example 1 the applicants outline their preferred method of producing limonen-10-ol from limonene while Example 2 shows the preferred method of producing menth-1-en-9-ol from the intermediate compound limonen-10-ol. In Example 3 formulations using menth-1-en--ol and limonen-10-ol as bactericides and fungicides are presented.